S -1 Shared Memory

S -2 Motivation Shared memory allows two or more processes to share a given region of memory -- this is the fastest form of IPC because the data does not need to be copied between the client and server The only trick in using shared memory is synchronizing access to a given region among multiple processes -- if the server is placing data into a shared memory region, the client shouldn’t try to access it until the server is done Often, semaphores are used to synchronize shared memory access ( … semaphores will be covered a few lectures from now) not covered in Wang, lookup in Stevens (APUE)

S -3 shmget() shmget() is used to obtain a shared memory identifier: #include int shmget( key_t key, int size, int flag ); shmget() returns a shared memory ID if OK, -1 on error key is typically the constant “ IPC_PRIVATE ”, which lets the kernel choose a new key -- keys are non-negative integer identifiers, but unlike fds they are system-wide, and their value continually increases to a maximum value, where it then wraps around to zero size is the size of the shared memory segment, in bytes flag can be “ SHM_R ”, “ SHM_W ”, or “ SHM_R|SHM_W ”

S -4 shmat() Once a shared memory segment has been created, a process attaches it to its address space by calling shmat() : void *shmat( int shmid, void *addr, int flag ); shmat() returns pointer to shared memory segment if OK, -1 on error The recommended technique is to set addr and flag to zero, i.e.: char *buf = (char *) shmat( shmid, 0, 0 ); The UNIX commands “ ipcs ” and “ ipcrm ” are used to list and remove shared memory segments on the current machine The default action is for a shared memory segments to remain in the system even after the process dies -- a better technique is to use shmctl() to set up a shared memory segment to remove itself once the process dies ( … see next slide)

S -5 shmctl() shmctl() performs various shared memory operations: int shmctl( int shmid, int cmd, struct shmid_ds *buf ); cmd can be one of IPC_STAT, IPC_SET, or IPC_RMID : –IPC_STAT fills the buf data structure (see ) –IPC_SET can change the uid, gid, and mode of the shmid –IPC_RMID sets up the shared memory segment to be removed from the system once the last process using the segment terminates or detached from it — a process detaches a shared memory segment using shmdt( void *addr ), which is similar to free() shmctl() returns 0 if OK, -1 on error

S -6 Shared Memory Example char *ShareMalloc( int size ) { int shmId; char *returnPtr; if( (shmId=shmget( IPC_PRIVATE, size, (SHM_R|SHM_W) )) < 0 ) Abort( "Failure on shmget {size is %d}\n", size ); if( (returnPtr=(char*) shmat( shmId, 0, 0 )) == (void*) -1 ) Abort( "Failure on Shared Mem (shmat)" ); shmctl( shmId, IPC_RMID, (struct shmid_ds *) NULL ); return( returnPtr ); }

S -7 mmap() An alternative to shared memory is memory mapped i/o, which maps a file on disk into a buffer in memory, so that when bytes are fetched from the buffer the corresponding bytes of the file are read One advantage is that the contents of files are non-volatile Usage: caddr_t mmap( caddr_t addr, size_t len, int prot, int flag, int filedes, off_t off ); –addr and off should be set to zero, –len is the number of bytes to allocate –prot is the file protection, typically ( PROT_READ|PROT_WRITE ) –flag should be set to MAP_SHARED to emulate shared memory –filedes is a file descriptor that should be opened previously

S -8 Memory Mapped I/O Example char *ShareMalloc( int size ) { int fd; char *returnPtr; if( (fd = open( "/tmp/mmap", O_CREAT | O_RDWR, 0666 )) < 0 ) Abort( "Failure on open" ); if( lseek( fd, size-1, SEEK_SET ) == -1 ) Abort( "Failure on lseek" ); if( write( fd, "", 1 ) != 1 ) Abort( "Failure on write" ); if( (returnPtr = (char *) mmap(0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0 )) == (caddr_t) -1 ) Abort( "Failure on mmap" ); return( returnPtr ); }

S -9 Semaphores

S -10 Motivation Programs that manage shared resources must execute portions of code called critical sections in a mutually exclusive manner. A common method of protecting critical sections is to use semaphores Code that modifies shared data usually has the following parts: Entry Section: The code that requests permission to modify the shared data. Critical Section: The code that modifies the shared variable. Exit Section: The code that releases access to the shared data. Remainder Section: The remaining code.

S -11 The Critical Section Problem The critical section problem refers to the problem of executing critical sections in a fair, symmetric manner. Solutions to the critical section problem must satisfy each of the following: Mutual Exclusion: At most one process is in its critical section at any time. Progress: If no process is executing its critical section, a process that wishes to enter can get in. Bounded Waiting: No process is postponed indefinitely. An atomic operation is an operation that, once started, completes in a logical indivisible way. Most solutions to the critical section problem rely on the existence of certain atomic operations

S -12 Semaphores A semaphore is an integer variable with two atomic operations: wait and signal. Other names for wait are down, P, and lock. Other names for signal are up, V, unlock, and post. A process that executes a wait on a semaphore variable S cannot proceed until the value of S is positive. It then decrements the value of S. The signal operation increments the value of the semaphore variable. Some (flawed) pseudocode: void wait( int *s ) void signal( int *s ) { { while( *s <= 0 ) ; (*s)++; (*s)--; } }

S -13 Semaphores (cont.) Three problems with the previous slide’s wait() and signal() : –busy waiting is inefficient –doesn’t guarantee bounded waiting –“ ++ ” and “ -- ” operations aren’t necessarily atomic! Solution: use system calls semget() and semop() (… see next slide) The following pseudocode protects a critical section: wait( &s ); /* critical section */ signal( &s ); /* remainder section */ What happens if S is initially 0? What happens if S is initially 8?

S -14 semget() Usage: #include int semget( key_t key, int nsems, int semflg ); Creates a semaphore set and initializes each element to zero Example: int semID = semget( IPC_PRIVATE, 1, S_IRUSR | S_IWUSR ); Like shared memory, icps and ipcrm can list and remove semaphores

S -15 semop() Usage: int semop( int semid, struct sembuf *sops, int nsops ); Increment, decrement, or test semaphores elements for a zero value. From : sops->sem_num, sops->sem_op, sops->sem_flg; If sem_op is positive, semop() adds value to semaphore element and awakens processes waiting for the element to increase if sem_op is negative, semop() adds the value to the semaphore element and if < 0, semop() sets to 0 and blocks until it increases if sem_op is zero and the semaphore element value is not zero, semop() blocks the calling process until the value becomes zero if semop() is interrupted by a signal, it returns -1 with errno = EINTR

S -16 Example struct sembuf semWait[1] = { 0, -1, 0 }, semSignal[1] = { 0, 1, 0 }; int semID; semop( semID, semSignal, 1 ); /* init to 1 */ while( (semop( semID, semWait, 1 ) == -1) && (errno == EINTR) ) ; { /* critical section */ } while( (semop( semID, semSignal, 1 ) == -1) && (errno == EINTR) ) ;